1. Field of the Invention
The field of the invention relates to topical anesthesia techniques, and applicators for providing anesthetic agents to the eye.
2. Brief Description of the Prior Art
For many years it has been traditional to use retrobulbar anesthesia when performing eye surgery. This technique has been described as early as 1884. While complications of retrobulbar anesthesia are few, they can be quite serious. These complications include: retrobulbar hemorrhage, respiratory depression, intradural or subarachnoid injection, optic nerve damage and perforation of the eye. In the early 1980's, in hopes of eliminating serious complications, many surgeons began to use periocular anesthesia with hopes that this technique would be a much safer one. In 1992, a number of eye surgeons in the United States began using topical anesthesia (no injections around the eye) for cataract surgery, which now totally eliminates the complications of the needle.
Retrobulbar anesthesia depends on a small volume (2-4 cc) of anesthesia being injected into the muscle cone directly behind the eye. In order to do this, a long needle (11/4"-11/2") is used. The patient is typically advised to look superior nasally while the needle is placed along the outer third of the lower lid. Once the needle penetrates the skin for about 1 cm, the needle is directed upward and nasal into the muscle cone. This also places the needle close to the posterior portion of the eye and near the optic nerve. Two to four cubic centimeters of anesthetic are injected at this point. Because the needle is directed upward and inward, there is a chance, especially in long myopic eyes, that the needle could penetrate the globe.
Another possible serious complication is optic nerve damage. Optic nerve damage can occur if too much anesthetic is injected in the retrobulbar space causing compression of the optic nerve. This could also occur if the needle penetrates the optic nerve sheath allowing the anesthetic to be injected directly along the optic nerve sheath. The latter would cause direct compression of the optic nerve. Another mechanism of optic nerve damage could occur if the needle is placed directly into the optic nerve. Compression of the optic nerve could result in central retinal vein occlusion, and either of these complications could result in optic atrophy and total blindness. Injecting anesthetic into the optic nerve sheath could allow the anesthetic to enter the subarachnoid or subdural space, resulting in respiratory depression and even transient contralateral blindness as well as mid-brain anesthesia. In order to avoid many of these complications, it has been suggested that a dull needle be used, but this has still not eliminated the problem.
Periocular anesthesia has been employed in the hope of eliminating some of the potentially serious complications of retrobulbar anesthesia. One periocular anesthesia technique utilizes a shorter, 26 gauge needle having a length of 3/4"-1". The needle is deliberately directed away from the eye and the anesthetic is deliberately injected outside the muscle cone. In order for this technique to be successful, a much larger volume of anesthetic (8-10 cc) is required. Because the anesthetic must disperse around the orbit, the surgeon must allow 12-20 minutes for it to take effect.
Even though periocular anesthesia is safer than retrobulbar anesthesia, and even though it has been documented that this technique reduces the incidence of complications, they can still occur if the needle is inadvertently placed in the wrong direction. Though complications with periocular anesthesia are rare, they can be serious and can even result in total loss of vision or loss of the eye.
By using topical anesthesia, any risk associated with the needle is eliminated. Topical anesthesia is an effective and reliable method of obtaining ocular anesthesia and gives the patient the benefits of increased safety, rapid return of vision with no loss of ocular motility and little risk of ptosis or double-vision. The anesthetic agents, e.g. tetracaine or Xylocaine, are typically applied to the eye as drops and/or with a sponge which has been soaked with the selected agent.
A ring-shaped structure having an inner diameter of 12 mm and an external diameter of 17.5 mm has also been available as an applicator for anesthetic agents. This applicator is made from polyvinyl alcohol, and includes a wick extension extending from the ring-shaped body. It has not gained wide acceptance in the field of ophthalmic surgery, however, and has a potentially serious drawback in that the inner diameter of the ring-shaped body is not sufficiently large to keep the topical anesthetic agents away from the limbus. The anesthetic is in direct contact with the corneal epithelium when the applicator is used, which can result in toxic effects. The extension may also allow the anesthetic to drain off from the area requiring anesthetization.